The present invention relates to frequency compensation of electronic circuits, and in particular to circuits with a compensation capacitor such as a Miller Effect capacitor.
A variety of electronic circuits require frequency compensation to enhance stability of the circuit. One such circuit is a low dropout voltage regulator (LDO). An LDO may be used in a variety of electronic devices such as cell phones, lap top computers, personal digital assistants, and the like. The LDO should be small in size, use less or cheaper external components, while exhibiting good regulation and fast transient response.
The compensation technique for the LDO can affect such factors. One compensation approach utilizes external compensation. This eliminates the need for internal compensation but also poses some problems. First, the external dominant pole shifts to a higher frequency when the load current increases. Second, the second non-dominant pole has to be shifted to a much higher frequency. A buffer with low output impedance could be used to achieve this, at the expense of a larger bias current. Further compromise has to be made using a capacitor having an appropriate equivalent series resistance (ESR) to provide a sufficient phase margin. These potential solutions, however, only work suitably if the LDO does not source a large load current.
Another compensation approach utilizes internal compensation. Such an internal compensation technique known in the art takes advantage of the Miller Effect by adding a Miller compensating capacitor to accomplish xe2x80x9cpole splitting.xe2x80x9d This approach generates a first dominant pole and a second non-dominant pole and has an associated unity gain frequency. However, a certain relationship must exist between the unity gain frequency and the second non dominant pole in order to maintain a minimum phase margin for the LDO.
For example, if the LDO requires a minimum phase margin of 60 degrees, the unity gain frequency would have to be at least a factor of two lower than the second non-dominant pole. This relationship between the unity gain frequency and the second non-dominant pole depends, in part, on the relationship between the Miller capacitance level and the load capacitance level. As the load capacitance level gets higher, so too must the Miller capacitance level. However, a larger Miller capacitor occupies more space in an environment where there is a premium on such space.
Accordingly, there is a need in the art for a compensating circuit for compensating circuits such as an LDO, where the compensating circuit includes a multiplying circuit for reducing the size of the Miller capacitor.
A compensation circuit for providing a compensating capacitive current to a regulating circuit consistent with the invention includes: a capacitor; and a multiplying circuit configured to accept an input capacitive current provided from the capacitor and multiply the input capacitive current by a predetermined multiplication factor to provide the compensating capacitive current, wherein the predetermined multiplication factor is based on a resistor ratio.
An LDO consistent with the invention includes: a regulating circuit having an input terminal, an output terminal, and a control terminal, the regulating circuit configured to receive an input signal at the input terminal and provide an output signal at the output terminal; a control circuit coupled to the control terminal of the regulating circuit, and configured to control the regulating circuit; and a compensating circuit for providing a compensating capacitive current to the regulating circuit, the compensating circuit including a capacitor and a multiplying circuit configured to accept an input capacitive current provided from the capacitor and multiply the input capacitive current by a predetermined multiplication factor to provide the compensating capacitive current, wherein the predetermined multiplication factor is based on a resistor ratio.
Another LDO consistent with the invention includes: a regulating circuit having an input terminal, an output terminal, and a control terminal, the regulating circuit configured to receive an input signal at the input terminal and provide an output signal at the output terminal; and a control circuit coupled to the control terminal of the regulating circuit, and configured to control and compensate the regulating circuit, the control circuit including a multiplying circuit configured to accept an input capacitive current from a capacitor and multiply the input capacitive current by a multiplication factor, wherein the multiplication factor is based on a resistor ratio.
An electronic device consistent with the invention includes: a power source configured to provide an unregulated power signal; and at least one LDO coupled to the power source for providing a regulated voltage signal to an associated load of the device, wherein the LDO includes: a regulating circuit having an input terminal, an output terminal, and a control terminal, the regulating circuit configured to receive the unregulated power signal from the power source at the input terminal and provide the regulated voltage signal at the output terminal; and a control circuit coupled to the control terminal of the regulating circuit, and configured to control and compensate the regulating circuit, the control circuit including a multiplying circuit configured to accept an input capacitive current from a capacitor and multiply the input capacitive current by a multiplication factor, wherein the multiplication factor is based on a resistor ratio.
A method of compensating a regulating circuit wherein the regulating circuit has an output terminal and a control terminal, the method consistent with the invention includes the steps of: receiving an input capacitive current through a capacitor coupled to the control terminal of the regulating circuit; and multiplying the input capacitive current by a multiplication factor to provide the compensating capacitive current to the control terminal of the regulating circuit, wherein the multiplication factor is based on a resistor ratio.
According to another aspect of the invention, there is provided a compensating circuit for providing a compensating voltage gain to a regulating circuit. The compensation circuit includes a voltage gain stage circuit configured to accept a first voltage signal representative of an output voltage level of the regulating circuit and multiply the first voltage signal by a predetermined multiplication factor to provide a second voltage signal, and a capacitor configured to accept the second voltage signal and provide the compensating voltage gain to the regulating circuit.
According to a further aspect of the invention, there is provided a LDO including a regulating circuit having an input terminal, an output terminal, and a control terminal. The regulating circuit is configured to receive an input voltage signal at the input terminal and provide an output voltage signal at the output terminal. The LDO also includes a control circuit coupled to the control terminal of the regulating circuit, and configured to control the regulating circuit, and a compensation circuit coupled to the control terminal of said regulating circuit and configured to provide a compensating voltage gain to the regulating circuit. The compensating circuit includes a voltage gain stage circuit configured to accept a first voltage signal representative of the output voltage signal of the regulating circuit and multiply the first voltage signal by a predetermined multiplication factor to provide a second voltage signal; and a capacitor configured to accept the second voltage signal and provide the compensating voltage gain to the regulating circuit.
According to yet a further aspect of the invention there is provided an electronic device having a power source configured to provide an unregulated power signal; and at least one LDO coupled to the power source for providing a regulated voltage signal to an associated load of the device, wherein the at least one LDO includes: a regulating circuit having an input terminal, an output terminal, and a control terminal, the regulating circuit configured to receive an input voltage signal at the input terminal and provide an output voltage signal at the output terminal; a control circuit coupled to the control terminal of the regulating circuit, and configured to control the regulating circuit; and a compensation circuit coupled to the control terminal of the regulating circuit and configured to provide a compensating voltage gain to the regulating circuit, the compensation circuit including a voltage gain stage circuit configured to accept a first voltage signal representative of the output voltage signal of the regulating circuit and multiply the first voltage signal by a predetermined multiplication factor to provide a second voltage signal, and a capacitor configured to accept the second voltage signal and provide the compensating voltage gain to the regulating circuit.
Finally, there is provided a method of compensating a regulating circuit wherein the regulating circuit has an output terminal and a control terminal, the method including the steps of: receiving a first voltage signal representative of an output voltage at the output terminal of the regulating circuit; multiplying the first voltage signal by a predetermined multiplication factor to provide a second voltage signal; and providing the second voltage signal to a capacitor, the capacitor in turn provides a compensating voltage gain to the control terminal of the regulating circuit.